1. Field of the Invention
The present invention relates to an electromagnetic clutch for intermittently transmitting the turning force of an external driving source such as an engine to a driving machine such as a compressor.
2. Description of the Related Art
Conventionally, as an electromagnetic clutch of this type, an electromagnetic clutch described, for example, in Japanese Utility Model Publication 7-35830 has been known.
The electromagnetic clutch described in this Publication is constructed so as to have an electromagnetic coil, a rotor rotated by an external driving source, an armature plate facing to the rotor with a clearance provided therebetween, a connecting member connected to a main shaft, and a plate spring for connecting the armature plate to the connecting member.
When the electromagnetic coil is in a deenergized state, the elastic force of the plate spring is given so as to separate the armature plate from the rotor, so that the turning force of the rotor is not transmitted to the armature plate.
On the other hand, when the electromagnetic coil is energized, the armature plate is attracted to the rotor against the elastic force of the plate spring, so that the armature plate is connected to the rotor. By the connection of the two elements, the turning force of the rotor is transmitted to the armature plate, and then the turning force of the armature plate is transmitted to the connecting member via the plate spring. As a result, the turning force of the connecting member is transmitted to the main shaft.
In the case where the external driving source is an automotive engine, and the main shaft is a rotating shaft of a compressor, the compressor is operated intermittently by the intermittent engagement of the electromagnetic clutch.
However, in the above-described conventional electromagnetic clutch, the connection force between the armature plate and the rotor relies on only the magnetomotive force of the electromagnetic coil. As a result, when a compressor with a high torque capacity is manufactured, it is difficult to reduce the weight of electromagnetic clutch and to save electric power because the electromagnetic coil and the rotor inevitably become large in size. Also, since a larger size of the electromagnetic coil increases the self-inductance, the clutch disengages poorly. Therefore, when the armature plate separates from the rotor, a very loud separation sound is emitted. Further, since the magnetomotive force of the electromagnetic coil is large, when the armature plate is connected to the rotor, the time taken until the synchronization of the armature plate with the rotor is short (the armature plate is connected to the rotor in an instant). As a result, a problem arises in that the turning force of the rotor is transmitted to the armature plate as an impact force, so that an adverse influence is exerted on the internal parts of the compressor.